The present invention generally relaing apparatuses for passive type display unparticularly, to a lighting apparatus which employs a light guide used in a back light for a liquid crystal display (LCD).
Generally, display units can be classified into active (luminous) type display units and passive (nonluminous) type display units. The active type display units are display units arranged to perform display by emitting light by themselves and include a light emitting diode display, a plasma display, an electroluminescent display, etc. On the other hand, the passive type display units are display units arranged to perform pattern display by modulating ambient light or irradiated light from other light sources through control of transmittance or reflectance of light without emitting light by themselves and include a liquid crystal display (LCD), an electrochromic display, etc. Accordingly, the passive type display units have a drawback when they are in a dark environment because their display becomes obscure unless a lighting means is provided.
The LCDs have such features as having small power consumption and being extensively used for portable electronic appliances such as an electronic calculator, an electronic wrist watch, etc. However, in most of the LCDs, since display modes are based on optical anisotropy, polarizing plates are strictly required to be provided so as to be piled on each other and thus, about 50% of irradiated light is absorbed by the polarizing plates. Especially, in an LCD employing a color filter, since intensity of light is further reduced for color display, it becomes indispensably necessary to provide the LCD with a lighting means for enabling indoor general use of the LCD. In the portable electronic appliances, since a power source is restricted in capacity, such a problem arises in manufacture of the LCDs that it is necessary to obtain a lighting apparatus capable of performing brighter lighting with smaller power consumption.
FIG. 1 shows a prior art back light used as the above described internal light source. The known back light includes a light source 1, a display panel 10, a diffuse transmission plate 4 disposed between the light source 1 and the display panel 10 and a reflector 5. A miniature incandescent lamp or a fluorescent lamp is used as the light source 1. The diffuse transmission plate 4 is made of milky glass or synthetic resin and has an effect of scattering light so as to uniform luminance of an illuminated face over a whole area of the illuminated face. Meanwhile, a reflector having a mirror surface or diffusive reflection surface for scattering light is used as the reflector 5. The reflector 5 is provided for reflecting light emitted rearwardly from the light source 1 so as to introduce the light forwardly such that a utilization factor of the light is improved. However, in the known back light referred to above, the light source 1 and the diffuse transmission plate 4 are required to be spaced a certain distance away from each other in order to lessen non-uniformity of brightness, thereby undesirably increassing depth of the known back light.
Furthermore, an edge light as shown in FIG. 2 has been conventionally used. The prior art edge light includes a light guide 6 and a display panel 10. The light guide 6 has faces A, B and C and the face B is roughly frosted. Light incident, at the face C, upon the light guide 6 from the light source 1 is subjected to diffuse reflection on the face B and then, is irradiated, through the face A, onto the display panel 10 disposed adjacent to the face A. This prior art edge light has such a drawback that luminance of the face B decreaset as a point on the face B is spaced further away form the light source 1.
Referring to FIG. 3, there is shown a known back light. The known back light includes a reflecting mirror 7 having a parabolic reflective surface. It is to be noted that in the case where the light source 1 is of a tubular shape, the reflective surface of the reflecting mirror 7 has a shape of a parabolic cylinder. In the known back light, light reflected on the reflecting mirror 7 constitutes a substantially parallel luminous flux. However, it will be understood from FIG. 3 that the known back light is disadvantageous in that as a point on the reflective surface is spaced further away from the center of the reflective surface, luminous flux density drops. Furthermore, the known back light has such an inconvenience as its large depth.